1. Field of the Invention
The present invention relates to a zoom lens system, and particularly to a compact zoom lens system suitable as a taking lens, for example, in a lens-shutter camera.
2. Description of the Prior Art
Conventionally, most zoom lens systems for lens-shutter cameras consist of lens units that are each composed of two or more lens elements. In order to reduce the size and the cost of such cameras, it is essential to compose their lens units of as few lens elements as possible.
To achieve the above purpose, U.S. Pat. No. 5,327,290 proposes a zoom lens system consisting of, from the object side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power. In this zoom lens system, each lens unit is composed of two lens elements. Moreover, Japanese Laid-open Patent Application No. H3-15881 proposes a zoom lens system consisting of two lens units, one having a positive refractive power and the other having a negative refractive power. In this zoom lens system, the number of lens elements composing each lens unit is reduced by the use of the aspherical surfaces; specifically, the first lens unit is composed of two lens elements, and the second lens unit is composed of as few as one lens element.
However, the zoom lens system proposed in these Japanese Laid-open Patent Applications have a defect in that it cannot satisfactorily correct the chromatic aberration over the whole system, because the chromatic aberration within each lens unit cannot be corrected sufficiently at high zooming ratios.